1. Field of the Invention
The present invention relates to a method of injection molding thermoplastic materials from an injection molding machine including a slidable screw into a mold whose optimum pattern of the injection flow rate has been determined on and is known for another injection molding machine. The method includes transferring the defined optimum characteristic values for a first injection molding machine onto an injection molding machine to be used.
2. Description of the Prior Art
In a flexible injection molding operation, frequent mold changes are required. However, for another lot size, not always the same injection molding machine is available on which the favorable characteristic values have already been determined, particularly the optimum injection flow rate pattern. Rather, from the number of injection molding machines which are available at a given time, that machine which is most advantageous for a given purpose is selected. If the given mold is then used on another injection molding machine, usually the injection procedure must again be optimized in order to obtain molded articles having the same properties. These properties are usually influenced by the mold filling procedure, the afterpressure procedure, the material temperature and the like. By determining the optimum mold filling procedure for a first injection molding machine, a defined screw advancing and injection flow rate pattern is obtained. If the same mold is now used on another injection molding machine, it is necessary to determine an equivalent injection flow rate pattern which will result in the same optimum mold filling procedure for this particular mold.
After the mold filling procedure has been concluded, the afterpressure phase is initiated. The shift from injection procedure to the application of afterpressure may be carried out as a function of distance, time or pressure. Independently of the type of parameter chosen for carrying out the shift, the mold-related injection time or mold filling time is always the correct physical reference value.
It is known in the art to introduce the rate of injection along the travel distance of the screw into a control device and, if necessary, to provide a graphic representation of the rate of injection. Since this control is carried out as a function of time, the injection rate/distance pattern of the screw advancement can always be represented and processed without great difficulty as an injection rate/time pattern.
In a method known from German patent No. 3,524,310, a screw travel distance/time curve with the mold-related injection time is stored and, by standardizing the travel distance units, a conversion to optimum values for another machine is possible. However, the method of this German patent is relatively complicated and the control of the method is relatively difficult.
It is, therefore, the primary of the present invention to provide a method of determining in a quick, simple and automated manner, based on the optimum characteristic values determined for a given mold on a first machine, the corresponding optimum characteristic values for operating the mold on a second machine.